1. Field of the Invention
The performance of a wind machine as related to power output is a function of the efficiency at which the machine extracts energy from the windstream. This phenomenon can be expressed as C.sub.p =P.sub.o /P.sub.w where C.sub.p is the power coefficient or coefficient of performance, P.sub.o is the power output, and P.sub.w is the power in the windstream. Kinetic theory dictates that the maximum value for C.sub.p is 59%. Tha actual value is in part a function of the airfoil characteristics and configuration. For a given design, C.sub.p is also dependent upon the rotational velocity of the airfoil, measured at its radial extremity or tip, in relation to the free-flow wind speed. This relationship is commonly referred to as the tip-speed/wind-speed ration (TS/WS), or simply the tip-speed ratio.
Wind energy conversion systems that extract energy from the wind to produce electrical power often use self-excited alternators to supply alternating current (AC) to stand-alone resistance heating systems or to be rectified as an input for synchronous inverters. Generally, these systems will obtain maximum efficiency and power output from a given wind regime if their tip-speed ratio can be maintained at an optimum constant value over the range of wind speed encountered.
Since the wind speed at a given site tends to fluctuate, it is apparent that failure of an airfoil to react to such fluctuations would have a substantial impact on the operational efficiency of the machine. This invention relates to a control system for responding to changes in wind speed so as to maintain a machine's operation at substantially peak performance.
2. DESCRIPTION OF THE PRIOR ART
Recognition of the advantage of improving wind system efficiency by modifying wind system control has been demonstrated by a number of previous approaches. The variable-pitch or variable-geometry wind turbine has been used in the prior art both to control rotational speed of the wind turbine and to increase the amount of energy extracted from the wind. Control of rotational speed using variable-pitch wind turbines has been used in some prior art devices to provide a constant rotational speed for a wide range of actual wind speeds which allows an alternator coupled to the wind turbine to provide a fixed output frequency and voltage. Not only do such devices fail to extract an optimum amount of energy from the wind, the variable-pitch, variable-geometry wind turbine is expensive, requires complex mechanical devices for proper operation, and failure of the complex mechanical controls can cause high-rotational-speed-induced failure in high winds which can cause extensive damage to equipment and possible injury to individuals. Thus, for optimum reliability, simplicity, and cost, it is desirable to use a fixed-pitch or fixed-geometry wind turbine if such a device can be made to operate in an efficient manner.
Moran et al., U.S. Pat. No. 4,095,120, teaches a system adaptable for use with a fixed blade turbine for improving the efficiency of a wind-driven generator. This system includes a generator speed sensor which cooperates with appropriate circuitry for incrementally controlling the field current. In U.S. Pat. No. 3,974,395, Bright also shows a field control system for a wind-driven electrical generator. Bright employs a tachometer coupled to the impeller shaft for generating an output signal representative of wind velocity. This signal is applied to a field control circuit, thereby adjusting the field current of the generator. Korzeniewski in U.S. Pat. No. 4,146,264 describes a similar approach for varying the loading of a wind-driven electric generator by sensing the rotational speed of the generator and varying the field of the generator in a stepwise fashion. A limitation characteristic of the systems of Moran et al., Bright, and Korzeniewski is that the tachometer signal in each case is not necessarily indicative of the actual wind speed, insofar as the rotational speed of the impeller or shaft is influenced by the load on the generator and therefore the field current is not necessarily correlated to the optimum operational parameter of tip speed to wind speed.
Japanese patent, Kokai No. 52-57926, controls the generator excitation current in a wind-driven machine for charging batteries in response to signals representing the actual output current and the cube of the wind speed. Soderholm et al., U.S. Pat. No. 4,331,881, teach a method of maximizing the power extracted from the wind by exercising control over a generator field so as to maintain a nearly constant tip-speed ratio. In addition to the circuitry for measuring the rotational speed, this system requires a transducer for accurately measuring the wind speed. The rotational speed and wind speed are continuously monitored and compared to one another so as to cause an adjustment to the field voltage when an imbalance occurs.